fix: correct typed-nil handling (#1891)

update: ready for review.

====================================
the dependency relationship: #1890 <- this one.

---------

Co-authored-by: Morgan Bazalgette <[email protected]>

gnovm/pkg/gnolang/preprocess.go

@@ -1402,6 +1402,19 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 						if !constConverted {
 							convertConst(store, last, n, arg0, nil)
 						}
+
+						// check legal type for nil
+						if arg0.IsUndefined() {
+							switch ct.Kind() { // special case for nil conversion check.
+							case SliceKind, PointerKind, FuncKind, MapKind, InterfaceKind, ChanKind:
+								convertConst(store, last, n, arg0, ct)
+							default:
+								panic(fmt.Sprintf(
+									"cannot convert %v to %v",
+									arg0, ct.Kind()))
+							}
+						}
+
 						// evaluate the new expression.
 						cx := evalConst(store, last, n)
 						// Though cx may be undefined if ct is interface,

gnovm/pkg/gnolang/values_conversions.go

@@ -75,6 +75,11 @@ func ConvertTo(alloc *Allocator, store Store, tv *TypedValue, t Type, isConst bo
 GNO_CASE:
 	// special case for interface target
 	if t.Kind() == InterfaceKind {
+		if tv.IsUndefined() && tv.T == nil {
+			if _, ok := t.(*NativeType); !ok { // no support for native now
+				tv.T = t
+			}
+		}
 		return
 	}
 	// special case for undefined/nil source

gnovm/tests/files/convert4.gno

@@ -5,4 +5,4 @@ func main() {
 }
 
 // Error:
-// main/files/convert4.gno:4:10: cannot convert (undefined) to int
+// main/files/convert4.gno:4:10: cannot convert (const (undefined)) to IntKind

gnovm/tests/files/convert5.gno

@@ -7,4 +7,4 @@ func main() {
 }
 
 // Error:
-// main/files/convert5.gno:3:1: cannot convert (undefined) to int
+// main/files/convert5.gno:3:1: cannot convert (const (undefined)) to IntKind

gnovm/tests/files/types/cmp_iface_0_stdlibs.gno

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"errors"
+	"strconv"
+)
+
+type Error int64
+
+func (e Error) Error() string {
+	return "error: " + strconv.Itoa(int(e))
+}
+
+var errCmp = errors.New("XXXX")
+
+// special case:
+// one is interface
+func main() {
+	if Error(0) == errCmp {
+		println("what the firetruck?")
+	} else {
+		println("something else")
+	}
+}
+
+// Output:
+// something else

gnovm/tests/files/types/cmp_iface_3_stdlibs.gno

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"errors"
+	"strconv"
+)
+
+type Error int64
+
+func (e Error) Error() string {
+	return "error: " + strconv.Itoa(int(e))
+}
+
+var errCmp = errors.New("XXXX")
+
+// special case:
+// one is interface
+func main() {
+	if Error(1) == errCmp {
+		println("what the firetruck?")
+	} else {
+		println("something else")
+	}
+}
+
+// Output:
+// something else

gnovm/tests/files/types/cmp_iface_5_stdlibs.gno

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"errors"
+	"strconv"
+)
+
+type Error int64
+
+func (e Error) Error() string {
+	return "error: " + strconv.Itoa(int(e))
+}
+
+var errCmp = errors.New("XXXX")
+
+// special case:
+// one is interface
+func main() {
+	if errCmp == int64(1) {
+		println("what the firetruck?")
+	} else {
+		println("something else")
+	}
+}
+
+// Error:
+// main/files/types/cmp_iface_5_stdlibs.gno:19:5: int64 does not implement .uverse.error (missing method Error)

gnovm/tests/files/types/typed_nil_a.gno

@@ -0,0 +1,18 @@
+package main
+
+import "fmt"
+
+type integer int
+
+func main() {
+	// illegal conversion
+	// should not work
+	if integer(nil) == nil {
+		fmt.Println("integer is nil")
+	} else {
+		fmt.Println("integer is not nil")
+	}
+}
+
+// Error:
+// main/files/types/typed_nil_a.gno:10:5: cannot convert (const (undefined)) to IntKind

gnovm/tests/files/types/typed_nil_b.gno

@@ -0,0 +1,16 @@
+package main
+
+import "fmt"
+
+type integer *int
+
+func main() {
+	println(integer(nil))
+	fmt.Println(integer(nil))
+	fmt.Printf("%T\n", integer(nil))
+}
+
+// Output:
+// (nil main.integer)
+// <nil>
+// *int

gnovm/tests/files/types/typed_nil_c.gno

@@ -0,0 +1,12 @@
+package main
+
+import "fmt"
+
+func main() {
+	println(interface{}(nil) == (*int)(nil))
+	fmt.Printf("%T\n", interface{}(nil))
+}
+
+// Output:
+// false
+// <nil>